This invention relates to an electrohydraulic valve system for controlling a fluid motor, such as a double-acting cylinder.
It is well known to control a fluid motor with a spool valve which is pilot-pressure controlled by an electrically operated pilot valve. Such valves have been proposed for use in closed loop fluid motor position control systems. However, such spool-type valves are susceptible to contaminants in the hydraulic fluid. Furthermore, such control systems must be designed to provide for smooth and stable operation when the system is controlling an overrunning load, such as when the fluid motor is lowering a heavy load. When this is done, however, the resulting control system is undesirably sluggish when controlling an underrunning load, such as when the fluid motor is lifting heavy loads. Another drawback of such valve systems is that complicated spools or additional valves are necessary to provide an operational mode wherein the fluid motor is allowed to float.
As an alternative to spool-type valves, it has also been proposed to control a double-acting cylinder via a four, on-off type poppet valve arrangement controlled by a pair of solenoid-operated pilot valves. Such a four-valve arrangement can provide for bi-directional cylinder movement, as well as cylinder float and lock functions. However, such on-off valves can produce undesirable high pressures when operating in a system having large fluid flow rates. Furthermore, in systems with high inertia, such on-off valves are prone to produce system instabilities, such as overshoot. Therefore, it would be desirable to provide a stable, closed-loop control valve system having the functional flexibility which is characteristic of four-poppet type valve arrangements.